Fluid property detection system

ABSTRACT

Apparatus for detecting the magnitude of a property of a fluid including a body mounted on a torsion spring that is oscillated by a feedback of alternately an in-phase and an out-of-phase signal from an amplifier having a gain adequate to sustain the oscillations. If the in-phase signal has a frequency directly proportional to f n  and the out-of-phase signal has a frequency directly proportional to f h , then the magnitude of the property p is determined by the digital computation of ##EQU1## where K and B are constants. The property p may be any one of several properties including but not limited to viscosity.

BACKGROUND OF THE INVENTION

This invention relates to the detection of a property of a fluid, andmore particularly to viscosity or other fluid property magnitudedetection.

In the past, prior art viscosimeters have had relatively poor accuracy.

SUMMARY OF THE INVENTION

In accordance with the system of the present invention, theabove-described and other disadvantages of the prior art are overcome byoscillating a body in a fluid with an amplified feedback impressed upona driver with an in-phase and out-of-phase signal at alternate periodsof time. It has then been found that if the oscillation frequency isdirectly proportional to f_(n) and f_(h) in the two sets of alternateperiods, respectively, then the magnitude of a property of the fluid,including but not limited to viscosity, will be ##EQU2## where K and Bare constants.

In accordance with the invention, means are provided to compute p inaccordance with the above equation.

The above-described and other advantages of the present invention willbe better understood from the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which are to be regarded as merely illustrative:

FIG. 1 is a vertical sectional view through a drag body assembly, partlyin elevation;

FIG. 2 is a top plan view of the assembly shown in FIG. 1;

FIG. 3 is a transverse sectional view taken on the line 3--3 of theassembly shown in FIG. 1;

FIG. 4 is a broken away sectional view, partly in elevation,illustrating the mount of a piezoelectric crystal shown in FIGS. 1 and2;

FIG. 5 is a vertical sectional view, partly in elevation, of analternative embodiment of the present invention;

FIG. 6 is a block diagram of a system of the present invention employinga drag body assembly which may be of the type shown either in FIGS. 1,2, 3 and 4, or in FIG. 5;

FIG. 7 is a block diagram of a frequency multiplier shown in FIG. 6;

FIG. 8 is a block diagram of a digital phase shifter shown in FIG. 6;

FIG. 9 is a graph of a group of waveforms characteristic of theoperation of the digital phase shifter of FIG. 8;

FIG. 10 is a block diagram of a reversible counter shown in FIG. 6 witha rate multiplier;

FIG. 11 is a block diagram illustrating a portion of two registers and agating circuit shown in FIG. 10;

FIG. 12 is a block diagram of a triangle wave generator shown in FIG. 6;and

FIGS. 13 and 14 are graphs of groups of waveforms characteristic of theoperation of the system of the invention shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a drag body assembly 20 may beemployed which includes a cylindrical container 21 closed at its lowerend, and a torsion spring assembly 22 having a disk 23 fixed inside ofcontainer 21 at the top thereof, all being shown in FIG. 1.

Disk 23 may be threaded into container 21, if desired. If so, a fluidsample may be inserted into container 21 by unscrewing disk 23 andinjecting the sample into container 21 through the upper end thereof.

All of the structure shown inside container 21 and spaced from theinternal cylindrical wall of container 21 may be integrally made from asingle isotropic piece of metal or other material. However, there is oneexception to this statement. A cylindrical portion or drag body 24 has ahole 25 therethrough in which a permanent magnet 26 is fixed. Thelocation of permanent magnet 26 may be still better understood from theview of FIG. 3. A disk 27 is integral with disk 23. Four torsion strips28, 29, 30 and 31 connect disk 27 to a disk 28' fixed relative to thetorsion strips 28 to 31. Torsion strips 28 to 31 may also be observed inFIG. 2.

In FIGS. 1 and 2, a piezoelectric crystal is illustrated at 32.

In FIG. 1, the diameter of drag body 24 is illustrated at A'. In thesame FIG. 1, the inside diameter of container 21 is illustrated at B'.The inside diameter B' is greater than the outside diameter A' by anamount equal to 60 mils, if desired.

As shown in FIG. 1, container 21 is fixed. As shown in FIG. 3, apermanent magnet 33, a ferromagnetic core 34 and a coil 35 around core34 are fixed relative to container 21. The axis of permanent magnet 33,core 34 and coil 35 is perpendicular to the vertical axis of drag body24 and makes and angle θ with the axis of permanent magnet 26 equal to,for example, 45°.

As shown in FIG. 4, crystal 32 may be mounted in any conventional wayhaving an upper electrode 36, a lower electrode 37 and a bonding layer38 bonding the lower electrode 37 to disk 23.

Upper electrode 36 has an electrical lead 39 connected therefrom. Lowerelectrode 37 has an electrical lead 40 connected therefrom. Both leads39 and 40 are also shown in FIG. 1.

An alternative embodiment of the drag body assembly 20 is illustrated at41 in FIG. 5. The drag body assembly 41 may be identical to drag bodyassembly 20 shown in FIG. 1 except that disk 23' corresponding to disk23 in FIGS. 5 and 1, respectively, is provided with a hole 42therethrough into which a tube 43 is sealed. If desired, the insidediameter of tube 43 may be approximately equal to or exactly equal tothe spacing between each oppositely disposed pair of torsion strips 28and 30, and 29 and 31. The hole 42 in disk 23' in FIG. 5 also extendscompletely through disk 27' corresponding to disk 27 in FIG. 1.

Drag body assembly 41 also is provided with a container 21' which may beidentical to container 21 shown in FIG. 1, except that it is providedwith a hole 44 therethrough into which a tube 45 is sealed. Inaccordance with the foregoing, as viewed in FIG. 5, fluid may be slowlycirculated upwardly or downwardly in tubes 43 and 45.

Fluid can pass between the interior cylindrical surface of container 21and drag body 24 in the space therebetween, the same having larger andsmaller inside and outside diameters, respectively, at B' and A' shownin FIG. 1 and as described previously.

The system of the present invention is shown in FIG. 6 including a dragbody assembly 20" which may be identical to assembly 20 or assembly 41shown in FIGS. 1 and 5, respectively.

Assembly 20" has a piezoelectric crystal 32' corresponding, for example,to crystal 32 shown in FIG. 1, a coil 35' corresponding to coil 35 shownin FIG. 3. Assembly 20" with a differential amplifier 46, a squarer 47,a frequency multiplier 52, a divide-by-250 divider 63, either adivide-by-two divider D1 or a digital phase shifter 51, an electronicswitch 48, a triangle wave generator 49, and a power amplifier 50 form aclosed loop electromechanical oscillator wherein the gain is adequate tocause drag body 24, for example, in FIG. 3 to rotate back and forth oroscillate about its vertical axis, as viewed in FIG. 1. The peak-to-peakrotation is relatively small in amplitude (perhaps about one degree),and when the upper end of core 34 in FIG. 3 becomes positive, itprovides additional attraction for the south pole of permanent magnet26. Permanent magnet 26 provides permanent bias which may or may notallow the south pole of permanent magnet 26 to rise to or above theposition in which it is shown in FIG. 3. When the upper end of core 34becomes a south pole, both it and torsion spring assembly 22 tend toreturn permanent magnet 26 toward the position shown in FIG. 3, torsionspring assembly 22 resisting downward movement of the south pole ofpermanent magnet 26.

Coil 35' in FIG. 6 is supplied with an in-phase voltage when switch 48is in a position not shown. When switch 48 is in the position shown,coil 35' is supplied with a signal which is, for example, 45 electricaldegrees lagging the phase of output of amplifier 46.

The phase of the input to coil 35' when it lags the output of amplifier46, in electrical degrees, is independent of frequency because of theuse of digital phase shifter 51 shown in FIG. 6.

The said electromechanical oscillator is thus operated with a feedbackwithout a phase shift during, for example, one second, and with the saidphase shift for another second. This is done alternately so long as thesystem is in operation.

If desired, the output of a frequency multiplier 52 at a junction 53 maybe described as f_(n) when the said electromechanical oscillator isoperating without a phase shift. The output of frequency multiplier 52,when a phase shift exists in the oscillatory loop, may be described asf_(h).

Other components of the system of FIG. 6 are a 1-megahertz clock 53', adivide-by-one thousand divider 54, a divide-by-one thousand divider 55,a divide-by-two divider 56, an electronic switch 57, an inverter 58, adifferentiator 59, a flip-flop 60, a counter 61, and AND gate 62, adivide-by-two hundred fifty divider 63, a double-pole, double-throwelectronic switch 64, an up-down or reversible counter 65, a ratemultiplier 66, a rate multiplier 67, a switch matrix 68, an offsetdigital computer 69 having a switch matrix 70, and a display unit 71.

Dividers 54 and 55 are connected in succession from the output of clock53'. Divider 55 has an output lead 72 which is high for one second andlow for the following second, the same being a slow square wave. Outputlead 72 of divider 55 is connected to the input of inverter 58, controlsthe position of switch 64, controls the updating of counter 65, andcontrols the alternate modes of oscillation of the said oscillatory loopby controlling the position of switch 48.

It is a nuance of the operation of the system of FIG. 6 that f_(h) islarger than f_(n). The system is therefore cycled by control of switch64 to cause counter 65 to count f_(h) for one second in the updirection, and to count f_(n) down when switch 64 is in the position notshown, in this position, f_(n) being impressed upon the down input ofcounter 65. In this way, the updated minimum count stored in counter 65represents the difference between f_(h) and f_(n). The stored value isthen multiplied by the pulses appearing at the output lead 73 of switch57. However, switch 57 is not closed for a full second. The closure ofswitch 57 is directly proportional to the period of f_(n) (thereciprocal of f_(n)). The closure of switch 57 is controlled by the "0"output of flip-flop 60.

Inverter 58 and differentiator 59 reset flip-flop 60 on the falling edgeof the square wave output on lead 72 of divider 55. This is the end ofthe time period for f_(h). Note that a junction is provided at 74 whichis connected to junction 53 at the output of frequency multiplier 52.Moreover, a lead 75 is connected from junction 74 to one input of ANDgate 62. AND gate 62 also receives another input from the "0" output offlip-flop 60. Counter 61 then counts f_(n) pulses and automaticallyresets by a reset output 76 thereof. At the same time, counter 61 resetsflip-flop 60 to the "1" state.

Counter 61 is set to a count so that the reset pulse at output lead 76thereof will occur prior to one second from the output pulse ofdifferentiator 59. The range of the instrument will be determined by howshort counter 61 should be. However, the frequency shift is always, ornearly always, quite small. Squarer 47 is connected to a junction 77which is also connected to the input of frequency multiplier 52. Thefrequency of the signal appearing at junction 77 when the output offrequency multiplier 52 is f_(n) may be 200.000 hertz. The frequency ofthe signal appearing at junction 77 when f_(h) exists may be 202.500hertz.

Counter 61 may reset on the count of 100,000; 150,000; or more or less,but may be close to, but must be less than, a count which would causeswitch 57 to be closed for more than one second.

The number stored in counter 65 is then multiplied by the number ofpulses appearing on output lead 73 of switch 57 during the time thatswitch 57 is closed.

Switch matrix 68 in combination with rate multiplier 67 merely reducesthe number of pulses at an output lead 79 of rate multiplier 66 so thatthe output of rate multiplier 67 is a number of pulses equal to ##EQU3##The factor K is set by adjustment of switch matrix 68. The factor B inthe equation set forth hereinbefore is subtracted by computer 69 bysetting matrix 70. The output of computer 69 is impressed upon a displayunit 71.

Frequency multiplier 52 may, for example, multiply the frequency of thesquare wave appearing at junction 77 by a factor of 1,000.

Triangle wave generator 49 may be replaced with a sine wave generator.It also may be replaced with a number of other components. The purposeof triangle wave generator 49 is to get a reasonably smooth fundamentalcurrent through coil 35'. The current which triangle wave generator 49produces in coil 35' is illustrated at i1 in FIG. 13.

As can be determined by inspection, dividers 54 and 55 merely areemployed to produce a two-second period square wave on output lead 72 ofdivider 55 responsive to the output of clock 53'. Divider 56 reduces theunnecessarily large pulse rate at the output of clock 53' from whichdivider 56 is connected so that the input to rate multiplier 66 will notbe unnecessarily large. The clock rate may be reduced, divider 56eliminated, and the size of one or both of the dividers 54 and 55reduced, if desired.

In some cases, the size of counter 61 may be increased or decreased, asdesired.

The combination of the structures illustrated in FIG. 6 is new. Only afew of the individual components of the system of FIG. 6 are per se new.

Divider 63 reduces the unnecessarily large frequency appearing at theoutput of frequency multiplier 52 to provide an input for digital phaseshifter 51.

All of the differentiators disclosed herein may be of the typeillustrated at 80 shown in FIG. 8, the same being entirely conventional.Clock 53' may be entirely conventional. The same is true of dividers 54,55, 56 and 63.

The electronic switches 48, 57 and 64 may be entirely conventional. Thesame is true of flip-flop 60 and counter 61. The same is also true ofAND gate 62 shown in FIG. 6. The same is true of differential amplifier46, squarer 47 and power amplifier 50 shown in FIG. 6. The same is trueof frequency multiplier 52, reversible counter 65, rate multiplier 66,rate multiplier 67, switch matrix 68, computer 69, switch matrix 70 anddisplay unit 71. The same is also true of inverter 58.

Rate multipliers 66 and 67 may be identical to those disclosed in U.S.Pat. No. 3,878,374, or they may be otherwise conventional, as desired.The same is true of switch matrix 68, computer 69, switch matrix 70 anddisplay unit 71. That is, all of the components 66, 67, 68, 69, 70 and71 may be identical to the corresponding components disclosed in U.S.Pat. No. 3,878,374.

Counter 65 may be entirely conventional with or without updating. Forexample, see the numerous reversible counters in Handbook of LogicCircuits by John D. Lenk, (Reston Publishing Company, Inc., Reston,Virginia, copyright 1972).

OPERATION

In the operation of the system of FIG. 6, when the signal on output lead72 of divider 55 goes high, the switches 48, 57 and 64 assume thepositions shown. Counter 65 then counts up. This is true because switch48, in the position shown, connects the output of digital phase shifter51 to triangle wave generator 49, and f_(h) appears at the output offrequency multiplier 52, f_(h) being larger than f_(n).

When the signal on output lead 72 of divider 55 goes low, the output ofinverter 58 causes differentiator 59 to reset flip-flop 60, and AND gate62 then passes the f_(n) pulses from lead 75 to the input of counter 61.Due to the said reset of flip-flop 60 by differentiator 59, the "0"output of flip-flop 60 is high, and the connection 81 therefrom toswitch 57 closes the switch 57. Thus, at the same time counter 65 iscounting down, two registers therein, to be described, are employed tocount down and to store at the same time. The stored difference betweenfrequencies f_(h) and f_(n) is then multiplied by the pulses thenappearing on the output lead 73 of switch 57 which is connected to theinput of rate multiplier 66. The number of pulses impressed upon theinput of rate multiplier 66 is then directly proportional to the periodwhich is the reciprocal of f_(n). In the equation set forthhereinbefore, the division by f_(n) is then accomplished by multiplyingby the period of f_(n), such period being represented by the number ofthe input pulses to rate multiplier 66 in each successive group thereof.

For the operation of components 67, 68, 69, 70 and 71, see the saidpatent.

If the system of FIG. 6 is employed to indicate viscosity, display unit71 will do so.

The system of FIG. 6 is calibrated against a standard empirically.Calibration may be employed by using the method of least squares, ormore easily, and perhaps less accurately, by reading display unit 71 forfluids of two different viscosities, measuring f_(h) and f_(n) at bothof the viscosities, forming two simultaneous equations for the twodifferent viscosities, and solving for the two unknowns K and B.

For the use of simultaneous equations and calibrating equipmentsempirically, see U.S. Pat. No. 3,677,067.

Frequency multiplier 52 shown in FIG. 6 may be of the type shown in FIG.7 or may be otherwise entirely conventional. Frequency multiplier 52 isprovided with a voltage controlled oscillator (VCO) 82, a squarer 83, adivide-by-one thousand divider 84, a phase detector 85 and a low passfilter 86 connected in succession in that order. The output of low passfilter 86 is connected to the input of VCO 82 over a lead 87. The inputto the frequency multiplier 52 is impressed upon phase detector 85 overan input lead 88.

Digital phase shifter 51 is shown in FIG. 8 including an inverter 90,differentiator 80 and a divide-by-two divider 91 connected in successionin that order.

The waveforms of FIG. 9 relate to FIG. 8. The waveforms of FIG. 9 arelabeled in FIG. 8. The waveform e10 is the output of divide-by-twodivider D2 in FIG. 12. The product of the amount divider 91 (FIG. 7)divides by and the amount divider 63 (FIG. 6) divides by must equal 500because the frequency multiplier 52 in FIG. 6 multiplies by 1,000, andthe frequency at the output of divider D2 (FIG. 12) must be the samefrequency as the frequency of the waveform appearing at junction 77regardless of the position of switch 48 shown in FIG. 6. The same istrue of the divisor products of dividers D1 and 63 in FIG. 6. Note willbe taken that the output e10 has been phase shifted 45 degrees in FIG. 9because the trailing edge of input waveform e6 has been employed totrigger the leading edge of the waveform of e10.

Counter 65 and rate multiplier 66 are again shown in FIG. 10. Counter 65may or may not be conventional, as desired. Counter 65 employs aconventional logic circuit 92, a conventional counter-register 93connected therefrom, a conventional gating circuit 94 connected fromregister 93, a conventional storage register 95 connected from gatingcircuit 94, storage register 95 being connected to rate multiplier 66.

Counter 65 is reset on the leading edge of the output pulse of divider55 in FIG. 6 by a connection therefrom through a differentiator 96. Theoutput of differentiator 96 is connected to logic circuit 92 and gatingcircuit 94. As is conventional, gating circuit 94 is employed totransfer the contents of counter-register 93 to storage register 95. Thepulses appearing on the input lead to rate multiplier 66 from switch 57are multiplied by the number stored in register 95, as is conventional.

The transfer of the contents of register 93 to register 95 throughgating circuit 94 is demonstrated by a stage in each of the registers 93and 95 corresponding to each other, and NAND gates in gating circuit 94which accomplish the transfer function.

In FIG. 11, the output of differentiator 96 is impressed upon an inputlead 97 which is connected to one input of each of two NAND gates 98 and99 connected respectively to the set and reset inputs of a flip-flop100. NAND gate 99 also receives an input from the "0" output of aflip-flop 101. NAND gate 98 also receives an input from the "1" outputof flip-flop 101. Flip-flop 101 represents a stage in register 93 whichcorresponds with a stage in register 95 including flip-flop 100.

Triangle wave generator 49 is shown in FIG. 12 including an electronicswitch 102 operated by the output (pole) of switch 48 shown in FIG. 6.

In FIG. 12, a square wave having a regulated positive and negativevalue, the absolute value of each being regulated to an identical value,if possible, is impressed upon an input junction 103 of a conventionalintegrator 104 having a feedback capacitor 105, an amplifier 106 and anelectronic reset switch 107 operated by the output of a differentiator116. Triangle wave generator 49 has an input lead 109 connected from thepole of switch 48 shown in FIG. 6. In FIG. 12, input lead 109, as statedpreviously, is connected to control the position of switch 102, but alsoforms a junction 110 with a lead 111.

Divider D2, inverter 115 and differentiator 116 are employed to zerointegrator 104 and thereby to prevent drift. As shown, integrator 104 iszeroed each time the triangular wave output 113 thereof passes throughzero volts at 114. Zeroing may be performed less often, if desired.Zeroing may be employed once per cycle of waveform 113, or less often.Inverter 115 and a differentiator 116 are connected from lead 111 toreset switch 107.

In accordance with the foregoing, the leading and trailing edges of eachpulse received over input lead 109 is detected by differentiator 116 andpulses at those points in time are employed to close switch 107 at thetrailing edges of the pulses appearing on input lead 109.

In FIG. 13, when switch 48 is in the position not shown in FIG. 6, e1represents the square wave appearing at the output of switch 48 and onthe input lead 109 of triangle wave generator 49 shown in FIGS. 6 and12, respectively. The same waveform when switch 48 is in the positionsshown in FIG. 6 appears at e2. Although, as shown, the waveforms (e1 ande2) may have no time correspondence, they are shown in timecorrespondence to illustrate that the waveform which always appears atjunction 77 in FIG. 6 is 45 degrees out of phase with the output ofdivider D2 in FIG. 12 when switch 48 is in the position shown, whereasthe square wave appearing at junction 77 (FIG. 6) is in phase with theoutput of divider D2 (FIG. 12) when switch 48 is in the position notshown in FIG. 6. Note will be taken that the waveform e2 lags waveforme1 by 45 degrees in FIG. 13.

As stated previously, the output of switch 48 in its two differentpositions have no time correspondence because the switch 48 in FIG. 6 isin only one position at a time, obviously. The phases of the twodifferent output signals of switch 48 on the pole thereof as shown inFIG. 6 therefore are unrelated. Although the frequency of the squarewave appearing at junction 77 in FIG. 6 is always nearly the same, it isin fact the two different frequencies thereat by which viscosity iscalculated. The properties of a fluid other than viscosity may also becalculated in the same way as disclosed in FIG. 6.

The time dimension C' shown in waveform e1 may be substantially equal tothe time width of each pulse in waveform e2 in FIG. 13, although, asstated previously, there is a finite difference which is important, andwhich is employed to compute viscosity, et cetera. The pulse width C'may be, for example, 5 milliseconds or so.

When switch 48 in FIG. 6 is in the position shown, e1 may represent thewaveform appearing at junction 77, and e2 may represent the waveform atthe output of divider D2 in FIG. 12. The dimension F' in comparingwaveforms e1 and e2 in FIG. 13 may be equal to C' divided by 4.

The current in coil 35' may be similar regardless of the position ofswitch 48. It may have a shape as indicated at i1 shown in FIG. 13, forexample, when switch 48 is in a position not shown.

Over the period D' shown in FIG. 14, e3 is the train of pulses at theoutput of frequency multiplier 52 (f_(h)). During the period E', e₃ isthe train of pulses at the output of frequency multiplier 52 (f_(n)).

The pulses shown in e4 in FIG. 14 represent the output pulses of divider56 shown in FIG. 6. After counter 65 has counted up during a periodcorresponding to period D' but immediately prior thereto, and counter 65has counted down during a period corresponding to E' during a periodimmediately prior thereto, the difference contained in register 93 (FIG.10) is then transferred to register 95 at the beginning of period D'shown in FIG. 14. Counter 65 then counts up again at frequency f_(h),and the multiplication provided by rate multiplier 66 shown in FIG. 6 isnot performed until the beginning of period E' shown in FIG. 14. At thesame time, counter-register 93 will be counting down during period E' .

The pulses at e5 in FIG. 14 represent the output pulses of switch 57shown in FIG. 6 which are passed to the input of rate multiplier 66 inFIG. 6. The input pulses to rate multiplier 66 shown in FIG. 6 come ingroups because switch 57 is not closed except over a period which may beslightly less than the period E' shown in FIG. 14 or may besubstantially less than the period E'. The period during which one pulsegroup of the pulses e5 in FIG. 14 are produced depends upon the size ofcounter 61 shown in FIG. 6.

The word "switch" is hereby defined for use herein and for use in theclaims to mean any kind of switch, but preferably an electronic orelectrical switch. Further, the word "switch" is hereby defined hereinfor use herein and for use in the claims to mean preferably anelectronic switch.

The flow through container 21' shown in FIG. 5, and as describedhereinbefore, is preferably moderate, slow, or very slow.

Because the energization of coil 35 shown in FIG. 3 causes theoscillatory rotation of drag body 24, it will be appreciated thatcontainer 21 and perhaps all of the structures therein are preferablynon-magnetic except for permanent magnet 26 shown in FIGS. 1 and 3.

The equation set forth hereinbefore may be employed to measure densityif viscosity is maintained substantially or exactly constant. Similarly,in order to display viscosity at display unit 71 shown in FIG. 6, it canbe important to maintain the fluid in container 21, for example, shownin FIG. 1 at a constant temperature so as to maintain the density of thefluid therein constant.

Torsion spring assembly 22 shown in FIG. 1 may be provided with anyconventional torsion spring, torque tube or torque rod, if desired.

Also, as stated previously, one or more magnitudes of one or morerespective fluid properties may appear at the output of computer 69 inFIG. 6 and be used in combination with any one or more components forthe computation of other variables in a larger system. In addition, thesystem of FIG. 6 may be employed to indicate the magnitude of a fluidproperty at display unit 71, or display unit 71 may be replaced by aprocess controller or otherwise. This is true regardless of whatmagnitude of what fluid property is represented by the output ofcomputer 69 in FIG. 6.

The word "fluid" as used herein and for use in the claims is herebydefined to be a liquid unless the invention is useful in thedetermination of a fluid property of one or more or all gases.

It is important to note that digital phase shifter 51 shown in FIG. 6 isa phase shifter which shifts the phase of the square wave appearing atjunction 77 shown in FIG. 6 independent of the frequency of the squarewave appearing at the said junction 77.

The maximum rotary movement of drag body 24 in FIGS. 1 and 3 may or maynot be 1.0 degree.

What is claim is:
 1. In a system for producing an output directlyproportional to the magnitude of a property of a fluid, the combinationcomprising: an electromechanical oscillator including a body movablymounted in a manner to be at least partially immersed in a fluid, sensormeans to produce a first periodic signal responsive to oscillation ofsaid body, first means connected from said sensor means to receive saidfirst periodic signal, electrical drive means mounted to oscillate saidbody, said first means including an amplifier and supplying second andthird periodic signals having first and second frequencies,respectively, substantially equal to those of said first periodic signalat different corresponding times, said first means supplying said secondand third periodic signals to said electrical drive means to cause thesame to oscillate said body and to form a closed loop, said first meansproducing signals of frequencies f_(n) and f_(h), the gain of saidamplifier being adequate to cause said loop to have sustainedoscillations at one of said first and second frequencies directlyproportional to said frequencies f_(n) and f_(h) , said first meansincluding a phase shifter connected to receive a signal of said firstperiodic frequency, said first means applying said second and thirdperiod signals to said electrical drive means alternately on a timeshared basis, said second periodic signal, when applied, being in phasewith said first periodic signal, said third periodic signal, whenapplied, being out of phase with said first periodic signal; and secondmeans connected from said sensor means for producing an output p inaccordance with the equation ##EQU4## where K and B are constants. 2.The invention as defined in claim 1, wherein p is viscosity.
 3. Theinvention as defined in claim 2, wherein a hollow cylinder is provided,said body having an external cylindrical surface mounted in asubstantially fixed axial but rotatable angular position concentricallywithin said hollow cylinder, said hollow cylinder having an internalcylindrical wall, said hollow cylinder containing the fluid, said bodycylindrical surface being positioned contiguous to said internalcylindrical wall of said cylinder.
 4. The invention as defined in claim3, wherein said body is suspended in said fixed axial position from saidcylinder by a torsion spring, said drive means being electromagnetic. 5.The invention as defined in claim 4, wherein said phase shifter includesa digital phase shifter, a single-pole, double-throw (SPDT) switcheffectively having first and second contacts connected from said sensormeans to receive said signals of said frequencies f_(n) and f_(h),respectively, said sensor means including a frequency multiplierconnected to receive a signal directly proportional to said firstperiodic signal, said digital phase shifter being connected from saidfrequency multiplier.
 6. The invention as defined in claim 5, wherein areversible counter is provided, effectively a double-pole, double-throw(DPDT) switch, said DPDT switch being connected from the output of saidfrequency multiplier to said reversible counter to cause the output ofsaid reversible counter to be directly proportional to the difference(f_(h) - f_(n)), clock means having one output to operate said SPDT andDPDT switches in synchronism, first and second rate multipliersconnected in that order, a reset counter for f_(n) pulses, gate meansconnected from said reset counter and connected from another output ofsaid clock means to pass constant frequency pulses in number directlyproportional to the reciprocal of f_(n) to the input of said first ratemultiplier, said first rate multiplier being connected from saidreversible counter, a first switch matrix connected to said second ratemultiplier to set said constant K, an offset digital computer includinga second switch matrix to set said constant B, and utilization means,said offset digital computer and said utilization means being connectedin succession in that order from said second rate multiplier.
 7. Theinvention as defined in claim 2, wherein said phase shifter includes adigital phase shifter, a single-pole, double-throw (SPDT) switcheffectively having first and second contacts connected from said sensormeans to receive said signals of said frequencies f_(n) and f_(h),respectively, said sensor means including a frequency multiplierconnected to receive a signal directly proportional to said firstperiodic signal, said digital phase shifter being connected from saidfrequency multiplier.
 8. The invention as defined in claim 1, whereinsaid phase shifter includes a digital phase shifter, a single-pole,double-throw (SPDT) switch effectively having first and second contactsconnected from said sensor means to receive said signals of saidfrequencies f_(n) and f_(h), respectively, said sensor means including afrequency multiplier connected to receive a signal directly proportionalto said first periodic signal, said digital phase shifter beingconnected from said frequency multiplier.
 9. The invention as defined inclaim 8, wherein a reversible counter is provided, effectively adouble-pole, double-throw (DPDT) switch, said DPDT switch beingconnected from the output of said frequency multiplier to saidreversible counter to cause the output of said reversible counter to bedirectly proportional to the difference (f_(h) - f_(n)), clock meanshaving one output to operate said SPDT and DPDT switches in synchronism,first and second rate multipliers connected in that order, a resetcounter for f_(n) pulses, gate means connected from said reset counterand connected from another output of said clock means to pass constantfrequency pulses in number directly proportional to the reciprocal off_(n) to the input of said first rate multiplier, said first ratemultiplier being connected from said reversible counter, a first switchmatrix connected to said second rate multiplier to set said constant K,an offset digital computer including a second switch matrix to set saidconstant B, and utilization means, said offset digital computer and saidutilization means being connected in succession in that order from saidsecond rate multiplier.
 10. The invention as defined in claim 9, whereinp is viscosity.
 11. The invention as defined in claim 10, wherein ahollow cylinder is provided, said body having an external cylindricalsurface mounted in a substantially fixed axial but rotatable angularposition concentrically within said hollow cylinder, said hollowcylinder having an internal cylindrical wall, said hollow cylindercontaining the fluid, said body cylindrical surface being positionedcontiguous to said internal cylindrical wall of said cylinder.